What is a isotope and how are isotopes of the same element different?

An isotope is an element with a different number of neutrons within its nucleus.

As that is poorly worded, perhaps I can clarify.  Every element is determined by the number of protons within the nucleus.  Hydrogen has one proton.  If an atom contains exactly one proton, it is an atom of hydrogen.  But hydrogen may have no neutrons (standard element), or there may be one proton and one neutron (deuterium), or one proton and two neutrons (tritium).  Deuterium and tritium are isotopes of hydrogen.

Helium has two protons, and any atom with only two protons is helium.  But standard helium also has two neutrons, for a total mass of four.  (2p + 2n = 4).   Atoms of helium with only one neutron have a mass of 3, and we call He3 an isotope of helium.

The elements range from 1 (hydrogen) to 108 or more.  Number 92 is uranium, which has 92 protons and a mess of neutrons.  U238 is the most common isotope of uranium, but U235 is famous for its use nuclear weapons.  Both of these isotopes are unstable, meaning they decay into other elements.